Hair is a keratinous filiform appendage of skin with a thickness of 0.005-0.6 mm and a length of several millimeters to one and a half meters. The length and thickness of the hair depends on many factors: race and sex, age, location, etc.
Hair follicles are located everywhere on the surface of the human body, with the exception of some anatomical regions. So, the hair is absent on the palms and soles, the lateral and palmar surfaces of the fingers, on the red border of the lips, the glans penis, the clitoris, the labia minora and the inner surface of the large labia.
Allocate different types of hair depending on their location, length, thickness, degree of pigmentation, as well as the presence or absence of the brain substance. As for the division of hair into types or species, a single classification does not yet exist. In the Russian dermatological and morphological school, it is customary to distinguish three types: long, bristly and cannon.
Long - this is a thick, long, pigmented hair covering the scalp, the pubic area, axillary cavities after puberty. In men, long hair grows in the area of the beard, mustache and other parts of the skin.
Bristly hair is also thick and pigmented, but unlike long, they are much shorter. Hair of this type form eyebrows, eyelashes, are found in the external auditory canal and on the threshold of the nasal cavity. As in long, and in bristly hair, there is a brain substance.
Pushkovye - this is the most numerous, thin, short, colorless hair, covering almost all areas of the skin. Western European and American dermatological schools adhere to a somewhat different division of hair: two main types are distinguished - gun and terminal. Pushkin hair is soft, without cerebral matter, rarely pigmented and rarely reaching a length of more than 2 cm. Terminal hair is characterized as coarser, longer, often pigmented and having a brain substance. It is indicated that there are a large number of intermediate types. It is known that hair of various types can grow from the same hair follicle throughout life under the influence of many exogenous and endogenous factors. If in the prepubescent period terminal hair is limited only by the scalp, eyebrows and eyelashes, then after puberty their localization expands, which is due to the activity of sex hormones. Under the influence of androgens in the period of puberty, as well as with various endocrine diseases, canine hair can be transformed into long hair. Particularly pronounced sensitivity to androgens are characterized by gun hair in the region of the upper lip, chin, areola of the nipples of the mammary glands and located along the midline of the abdomen below the navel.
In addition to hormonal effects, the development of terminal hair is influenced by constitutional and racial factors, which are genetically determined.
Hair color depends on the activity of melanocytes and is caused by two pigments: yellow-red pheomelanin and black-brown eumelanin. Their biosynthesis occurs under the influence of genetic predisposition and the endocrine system. Variants of hair color depend on the combination of both pigments. Thus, black hair contains more eumelanin, and in light hair it contains a feomelanin. Red hair in subjects with white skin contains only pheomelanin. The color of light-brown hair is determined by a separate gene of different expressiveness.
The structure of the hair is also genetically determined and largely depends on the race. The most noticeable differences between races are noted on the scalp. Thus, the Mongoloid race is characterized by coarse, straight hair; for Negroid - rough, considerably crimped (spiraling and "woolly"), for Caucasoid - soft, slightly curly wavy hair. These types of hair have a different shape on the cut.
The hair consists of a stem protruding above the skin level and a root located in the hair follicle. Each follicle is a cylindrical epithelial formation (a kind of "invagination"), resembling a stocking and immersed in the depth of the dermis and hypodermis. The hair follicle is braided with a connective tissue bag consisting of the inner and outer layers. In the inner layer, the fibers are circular, and in the outer layer - longitudinally. Near the surface of the skin, the hair follicle forms an extension called the funnel. The funnel of the follicle flows into the duct of the sebaceous gland (in all areas of the skin), as well as the apocrine sweat gland (in the region of the axillae, the areola of the nipple glands on the chest, perianal, perigenital, etc.). At the end of the follicle there is an extension - a hair bulb, into which a connective tissue hair papilla grows with a large number of blood vessels that feed the hair bulb. Epithelial cells of the bulb are cambial elements that provide hair growth. They actively divide and, differentiating and forming (depending on the position in the bulb) cells of different types, which are subjected to keratinization and participate in the formation of various parts of the hair, as well as its inner root vagina. In the bulb there are melanocytes, which determine the pigmentation of the hair, as well as nerve endings.
The brain substance of the hair is formed by the cells of the central part of the bulb. It consists of weakly pigmented, vacuolated cells lying like coin pillars and containing in the cytoplasm oxyphilic granules of the trichogialine, the precursor of the horny substance. Cells of the brain substance completely keratinize only at the level of the sebaceous glands.
Cortical substance of the hair is formed by the middle part of the hair bulb. It is located around the brain substance and consists of flattened spindle-shaped cells, which rapidly horn, filling with hard keratin.
The cuticle of the hair is formed by the outer edge of the middle part of the bulb; surrounds the cortical substance and consists of cells, which turn into solid keratin-containing horny scales. They are tiled overlapping each other with projecting edges upward. It is the dense closing of horny scales that provides a natural shine of the surface of the hair and prevents its excessive moistening or loss of moisture. Dense closure of flakes is ensured by the double lipid interlayers between them, between which are hydrophilic substances, in particular ceramides.
The inner epithelial vagina is formed by the peripheral part of the bulb and surrounds the root of the hair to the level of the ducts of the sebaceous glands, where it disappears. It consists of three layers, which are clearly discernible only near the bulb and merge higher into a single corneous layer (from the inside, outwards):
- cuticle of the inner epithelial vagina - similar to the cuticle of the hair, its scales contain soft keratin. They are directed by their projecting edges downwards and are woven with scales of the cuticle of hair;
- The inner (granular) Huxley layer - near the bulb is formed by cells containing trichohialin granules, which, when displaced upward, are filled with soft keratin and break down;
- the outer (pale) Henle layer is formed by one row of light cubic cells filled with soft keratin and disintegrating.
The outer epithelial vagina is the continuation of the epidermis in the follicle. It loses the stratum corneum at the level of the sebaceous glands and, thinning to 1-2 layers, fuses with the bulb.
The muscle that lifts the hair consists of smooth muscle cells; she at one end is woven into a hair bag, and the other - into the papillary layer of the dermis. The muscle is innervated by the fibers of the autonomic nervous system.
Pushed hair is of great importance in the formation of tactile sensitivity, acting as a kind of "sensitive endings", with the stimulation of which there is a contraction of the muscle that lifts the hair. When it is cut, the obliquely lying hair approaches the vertical position, and the skin in the area of the muscle attachment is retracted. As a result, an underlined follicular pattern appears. This phenomenon is the basis of the pilomotor reflex, which characterizes the state of autonomic innervation. Hair, located on other sites, also perform a number of specific functions. So, for example, with the stimulation of the hair on the threshold of the nasal cavity, the act of sneezing occurs, and when the eyelashes are exposed, the eyelid closes.